In addition to their traditional propulsion functions, gas turbine engines are also used aboard aircraft as auxiliary power units (APU) to supply pneumatic power to a wide variety of accessory devices and systems. This is accomplished by bleeding a desired quantity of compressed air from a centrifugal "load" compressor which is connected to and driven by the engine's drive shaft.
Ambient air is drawn axially into the load compressor through the annular flow passage of an intake assembly which has a circular, radially outwardly facing inlet opening that circumscribes the drive shaft. Adjustable inlet guide vanes are mounted in a mutually spaced relationship around the circumference of the radial inlet opening for conjoint pivotal motion about axes parallel to the shaft axis between a fully closed position, in which the vanes are each generally tangentially disposed relative to their inlet opening, and a fully open position in which each of the vanes extends generally radially inwardly therefrom. By selectively adjusting the angular position of these vanes the flow rate of air entering the load compressor (and thus the flow rate of compressed air supplied to the pneumatically-operated accessory apparatus) during engine operation may be accurately regulated.
Because of their orientation relative to the drive shaft axis, the inlet guide vanes, within a certain range of opening angles, impart to air traversing the intake assembly flow passage a desirable vortex pattern in which the air swirls about the shaft axis as it is drawn axially into the load compressor. This vortex pattern causes the air therein to contact the curved impeller blades of the centrifugal load compressor at an efficient angle of incidence.
However, in conventional radial-to-axial air intake assemblies of the type described, the induced air swirl also creates, at certain inlet guide vane angles, a shrill intake noise known as vortex whistle or the Ranque-Hilsch effect. Vortex whistle is undesirable from two standpoints. First, it is often unacceptable under applicable acoustic standards. Second, generation of the whistle within the intake assembly causes an aerodynamic energy loss which diminishes the efficiency of the load compressor.
U.S. Pat. No. 4,844,695 discloses one approach for attenuating or eliminating vortex whistle in a centrifugal compressor inlet. This approach employs a plurality of flow fences disposed along the radially inner wall between the inlet guide vanes and the compressor and extending into the flow path. These fences apparently attenuate the vortex whistle by disrupting a portion of the swirling air flow generated by the inlet guide vanes.
Another approach to attenuating vortex whistle is disclosed in U.S. Pat. Nos. 4,436,481, 4,439,104, and 4,531,356 which are assigned to the assignee of this application. With this approach, elongated tabs are mounted on a pair if diametrically opposed inlet guide vanes. The tabs are rotatably mounted to the leading edges of the guide vanes. As the vanes close, the tabs extend into the flow path where they create small zones of random turbulence which attenuate the vortex whistle. Though this approach has been successfully used on numerous engines, on some engines it has been discovered that the insertion of the elongated tabs into the flow path altered the inlet guide vane angle at which the vortex whistle occurred.
Accordingly, there is a need for an apparatus and method that eliminates or minimizes vortex whistle regardless of the inlet guide vane angle.